This application claims the benefit of Japanese Patent Applications No. 2003-019716 filed Jan. 29, 2003, No. 2003-071302 filed Mar. 17, 2003, No. 2003-081836 filed Mar. 25, 2003, No. 2004-011536 filed Jan. 20, 2004, No. 2004-013562 filed Jan. 21, 2004, and No. 2004-012906 filed Jan. 21, 2004, in the Japanese Patent Office, the disclosures of which are hereby incorporated by reference.
The present invention generally relates to methods of growing group III nitride crystals, group III nitride crystals grown thereby, group III nitride crystal growing apparatuses and semiconductor devices, and more particularly to a method of growing a group III nitride crystal which is suited for use in semiconductor devices such as violet light sources which may be used for writing and/or reading information to and/or from optical disks, ultraviolet light sources such as laser diodes and light emitting diodes, violet light sources which may be used for electronic photography, and group III nitride electron devices. The present invention also relates to a group III nitride crystal and a semiconductor device which are produced using the method of growing the group III nitride crystal, and to a group III nitride crystal growing apparatus for growing such a group III nitride crystal.
Existing InGaAlN (group III nitride) devices which are used for violet, blue and green light sources are generally made by using crystal growing methods such as Metal Organic Chemical Vapor Deposition (MO-CVD) and Molecular Beam Epitaxy (MBE) to grow the InGaAlN (group III nitride) crystal on a sapphire or SiC substrate. But when the sapphire or SiC substrate is used, crystal defects increase due to large differences between the coefficient of thermal expansion and the lattice constant of the InGaAlN (group III nitride) crystal and the coefficient of thermal expansion and the lattice constant of the sapphire or SiC substrate. As a result, device performances of the semiconductor devices having the InGaAlN (group III nitride) grown on the sapphire or SiC substrate deteriorate, thereby making it difficult to extend the serviceable life of the light emitting devices, for example, and increasing the power required to operate such semiconductor devices.
Furthermore, in the case of the sapphire substrate which is insulative, it is impossible to draw out the electrodes via the substrate as done in the conventional light emitting devices. Consequently, when the sapphire substrate is used, the electrodes must be drawn out via the group III nitride crystal layer which is grown on the sapphire substrate. As a result, the device area becomes large and it becomes difficult to reduce the cost of such semiconductor devices.
In addition, in the case of the semiconductor device having the group III nitride crystal grown on the sapphire substrate, it is difficult to separate the chips by slicing, and it is not easy to obtain a resonator end surface required by the laser diode by cleavage. For this reason, existing techniques form the resonator end surface by dry etching or, polishes the sapphire substrate to a thickness of 100 μm or less before forming the resonator end surface by a process similar to cleavage. But according to these existing techniques, it is impossible to easily perform the formation of the resonator end surface and the chip separation in a single process as done in conventional laser diodes. Accordingly, these existing techniques require complex processes and increase the cost of the semiconductor devices.
In order to solve the problems described above, a technique was proposed to reduce the crystal defects by taking measures such as selectively growing the group III nitride crystal in a lateral direction on the sapphire substrate. According to this proposed technique, it is possible to reduce the crystal defects compared to a case where a GaN layer is not selectively grown in the lateral direction on the sapphire substrate. However, the above described problems associated with the insulation and the cleavage caused by the use of the sapphire substrate still exist. Furthermore, this proposed technique requires complex processes, and the sapphire substrate warps due to the growing of the GaN layer on the sapphire substrate since sapphire and GaN have different properties. As a result, the cost of the semiconductor device becomes high when this proposed technique is employed to make the semiconductor device.
In order to solve these problems, it is desirable to grow on the substrate a layer which is made of the same material as the substrate. In the case described above, it is desirable to grow the GaN layer on a GaN substrate. Hence, research is being made to grow the crystal of bulk GaN by vapor phase deposition, solution growth and the like. However, a GaN substrate having a practical size and a high quality has yet to be realized.
One method of realizing the GaN substrate is proposed in H. Yamane et al., “Preparation of GaN Single Crystals Using a Na Flux”, Chem. Mater. 1997, Vol. 9, pp. 413-416. This proposed method grows the GaN crystal using Na as flux. More particularly, this proposed method uses NaN3 and Ga as raw materials, and seals the raw materials in a nitrogen atmosphere within a stainless steel reaction chamber which has an internal diameter of 7.5 mm and a length of 100 mm, for example. The GaN crystal is grown by maintaining the reaction chamber at a temperature of 600° C. to 800° C. for 24 hours to 100 hours.
In the case of the proposed method according to H. Yamane et al., the Ga crystal can be grown at a relatively low temperature of 600° C. to 800° C. In addition, the pressure within the reaction chamber is on the order of approximately 100 kg/cm2 and is relatively low. Hence, the growth condition of this proposed method is practical.
However, the problem with this proposed method is that the size of the obtained crystal is on the order of approximately 1 mm or less and small. In other words, the reaction chamber used in H. Yamane et al. is a completely closed system, and the raw materials cannot be supplied from outside the reaction chamber. For this reason, the raw materials are depleted during the crystal growth and the crystal growth stops, thereby making the size of the obtained crystal on the order of approximately 1 mm and small. From the practical point of view, the crystal having such a small size is unsuited for making the semiconductor device.
In view of the above, first and second methods were respectively proposed in Japanese Laid-Open Patent Applications No. 2001-58900 and No. 2001-102316.
FIG. 1 is a cross sectional view showing a crystal growing apparatus used by the first method. As shown in FIG. 1, a growth chamber 102 and a group III metal supply pipe 103 are provided within a reaction chamber 101. External pressure is applied to the group III metal supply pipe 103 from outside the reaction chamber 101, so as to additionally supply a group III metal 104 to the reaction chamber 102 which contains flux. In other words, in order to increase the size of the group III nitride crystal which is obtained, the first method additionally supplies the group III metal 104 when growing the group III nitride crystal.
The group III metal supply pipe 103 has a hole 105. The crystal growing apparatus further includes a pressure applying unit 106, an internal space 107 of the reaction chamber 101, a nitrogen supply pipe 108, a pressure control unit 109, a lower heater 110, and a side heater 111.
On the other hand, the second method may be categorized into a mixing method and a fusion method. The mixing method applies external pressure to a molten mixture supply pipe which contains a molten mixture of flux (Na) and group III metal (Ga), so as to additionally supply the molten mixture to a growth chamber which contains the flux. The fusion method supplies an intermetallic compound of flux (Na) and group III metal (Ga), and additionally supplies the group III metal by partial fusion of the intermetallic compound.
According to the first and second methods described above, the raw materials are additionally supplied during the crystal growth, thereby making it possible to grow larger crystals.
However, according to the first method, vapor of the flux (Na) concentrates at a low-temperature portion, causing the flux (Na) to adhere on the group III metal supply pipe 103 which has a low temperature. As a result, the hole 105 of the group III metal supply pipe 103 may be clogged by the adhered flux (Na). If the temperature of the group III metal supply pipe 103 is increased in order to prevent the flux (Na) from adhering thereon, the group III metal reacts with the material forming the group III metal supply pipe 103 in a case where the group III metal is Ga and the material forming the group III metal supply pipe 103 is stainless steel, for example. Consequently, the hole 105 of the group III metal supply pipe 103 is also clogged when such a reaction occurs between the group III metal and the material forming the group III metal supply pipe 103.
On the other hand, according to the mixing method of the second method, the flux exists within the molten mixture supply pipe. For this reason, the group III metal and the nitrogen react within the molten mixture supply pipe and generate a group III nitride, to thereby clog the molten mixture supply pipe.
According to the fusion method of the second method, if the intermetallic compound is mixed into the flux and partially fused, a rapid reaction occurs between the intermetallic compound and the nitrogen, to thereby deteriorate the crystal properties of the group III nitride which is obtained.
And, according to the mixing and fusion methods of the second method, the solubility of the nitrogen to the molten mixture is small, and the growth rate of the group III nitride crystal is low.